He bought a ticket, caught the right train and even managed to avoid the wrong type of commuter on the line.

After his previous scrape with public transport, Tony Blair found a novel way of persuading passengers to talk to him on London Underground yesterday: travel with Frank Dobson.

The last time the Prime Minister was on the Jubilee Line, he famously tried and failed to strike up a conversation with a Walkman-wearing office worker. Wary of another PR opportunity going down the Tube, Mr Blair wasn't taking any chances as he helped to launch Mr Dobson's transport manifesto at London Bridge station.

The subterranean stunt was the centrepiece of the Labour leader's attempts to scotch rumours that he has abandoned his official candidate for mayor.

Mr Blair decided to end weeks of "will-he-or-won't-he?" speculation when he finally came out of the closet and declared his undying support for the former health secretary.

Nearly two months have passed since Mr Dobson beat Ken Livingstone for the party's nomination and, judging from the relieved look on the Labour candidate's face, the Prime Minister's guest star appearance was more than welcome.

Before the two men went into the Underground station at mid-morning, they appeared at the Glaziers Hall, at London Bridge, to prove to the world just how close they were.

Dressed in a blue suit and sporting the frazzled look of a father-to-be, Mr Blair certainly didn't look like a man who wanted to distance himself from Mr Dobson.

After both men made brief speeches attacking Mr Livingstone, it was the Prime Minister who was on his feet, answering questions about every aspect of the mayoral campaign.

Mr Blair attacked the former GLC leader's plans to raise bonds for the Tube and to impose £100-a-month congestion charges on motorists.

He said that he would work with "whoever is the mayor", but stressed that under the Greater London Act it was the Government that would decide which system of investment was best for the Tube.

Mr Blair expressed his exasperation at the constant criticism: first that Mr Dobson was his "poodle" and, more recently, that he had ditched him in the face of bleak opinion polls.

"Frank has to wear the jibe that he was simply my placeman, and now it's the opposite. Come on, I've known this guy, I've worked with him for over 20 years," the Prime Minister said. "Of course the personality is important, but so is the political personality. This is a guy who worked 24 hours a day for London. His problem is getting you guys to focus on policy.

"I tell you what's going to happen in the next few weeks. People will start to see the problems of Ken Livingstone. There's only one serious alternative and that's this guy here."

All guyed out, the PM then decided to accompany Mr Dobson on a carefully planned "surprise" visit to the spanking new Jubilee Line, triggering a media scrum in the process.

With television and press cameramen in hot pursuit, Mr Blair bought his £1.50 single to Westminster, glided through the ticket barriers and commanded scores of pictures of him and his candidate on the escalators.

To the delight of Millbank spin-doctors, a mother and her two sons then appeared, as if by magic, to provide another cuddly photo opportunity. The family was on its way to Mr Blair's other pet project, the Millennium Dome. Unfortunately for Mr Dobson, all the questions were to the Prime Minister.

When one of the boys asked him whether he wanted a baby girl or a boy, Mr Blair even obliged with a scoopette for the willing youngster. "To be honest, I find girls easier," he confided.

Before you could picture tabloid headlines screaming "IT'S A GIRL!", the PM was off again, taking the three short stops to Westminster and Downing Street.

The last time he was on the Jubilee Line, he had to endure the embarrassment of being snapped next to a secretary who preferred to listen to Whitney Houston in stereo than Blairspeak in mono.

But with Mr Dobson on hand as a welcome foil, Mr Blair was not caught short without a friendly face this time and several strangers on the train smiled coyly.

However, one of the score of police officers on board the train remained unconvinced by the prime ministerial conversion to public transport. "At least he bought a ticket, unlike his wife," he said.

After apologising repeatedly to fellow passengers for the fuss caused by his presence, Mr Blair declared "Well, this is my stop," before hopping off at Westminster.

In a tear-jerking scene reminiscent of Brief Encounter, the Prime Minister then said his goodbyes. Mr Dobson headed off to the House of Commons, while the PM briskly, regally walked back to Downing Street.

After months of waiting, Tony's mayoral baby is almost due. The pre-natal scans indicate that it will be a Ken, but he clearly still hopes it will be a Frank after all.